You have to register any starbucks card you have, which enrolls you into their rewards program. If you register a card 30+ days before your birthday, you are eligible for a free drink on your birthday.

You have to get a certain # of "stars" to reach the first level (Green). The same goes for Gold. In order to maintain your level, you have to have a certain number of stars 12 months after you first get to that level. If you go to Starbucks a lot, this shouldn't be a problem. At green level, you get free refills on any brewed coffee or iced tea. At gold level, every 12 purchases you make earns you a free drink of your choice.

Stars can be earned by making purchases as well as completing certain star promotions (buy drinks 7 days in a row, get 7 extra stars and stuff like that). Purchases are indisscriminate. You get 1 star whether you buy a tall or a trente. Refills don't get you any stars.

Of course, in order to use this long term, you have to reload your card constantly. This is pretty easy if you have a smartphone.